Lithium ion rechargeable batteries are well known. The basic construction of a lithium ion rechargeable battery is shown in FIG. 1. The battery cell includes a single cell, but may include multiple cells.
The battery cell generally comprises a copper current collector 10 for the anode and an aluminium current collector 12 for the cathode, which are externally connectable to a load or to a recharging source as appropriate. It should be noted that the terms “anode” and “cathode” are used in the present specification as those terms are understood in the context of batteries placed across a load, i.e. the term “anode” denotes the negative pole and the term “cathode” the positive pole of the battery. A graphite-based composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12. A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and a lithium containing metal oxide-based composite cathode layer 16: a liquid electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16.
When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide in the cathode via the electrolyte into the graphite-based anode where it is intercalated by reacting with the graphite to create a lithium carbon compound, typically LiC6. The graphite, being the electrochemically active material in the composite anode layer, has a theoretical maximum capacity of 372 mAh/g.
The use of silicon as an active anode material in secondary batteries such as lithium ion batteries is well known (see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries, M. Winter, J. O. Besenhard, M. E. Spahr, and P. Novak in Adv. Mater. 1998, 10, No. 10 and also Wang, Kasavajjula et al, J. Power Source's 163 (2007) 1003-1039). It is generally believed that silicon, when used as an active anode material in a lithium-ion rechargeable cell, can provide a significantly higher capacity than the currently used graphite anode materials. Silicon, when converted to the compound Li21Si5 by reaction with lithium in an electrochemical cell, has a theoretical maximum capacity of 4,200 mAh/g, considerably higher than the maximum capacity for graphite.
Early approaches of using silicon or silicon based active anode materials in a lithium ion electrochemical cell included the use of bulk silicon anodes, silicon powder anodes comprising nanometer and micron sized silicon powders, thin film silicon anodes and silicon anodes comprising silicon structures other than or in addition to powders. Composite anodes comprising a dispersion of silicon in an inactive or active matrix material have also been investigated. However, many of the approaches have failed to show sustained or adequate capacity over the required number of charge/discharge cycles.
Electrodes comprising bulk silicon failed to exhibit good capacity retention and cycle-ability over a number of charging and discharging cycles. This poor performance was attributed to the mechanical stresses that arise within the electrode structure during the charging cycle. Intercalation or insertion of lithium ions into the bulk silicon structure during the charging cycle causes a massive expansion of the silicon containing material, which leads to a build-up of mechanical stress within the electrode structure and eventually causes cracking, delamination and loss of contact within and between the components of the electrode structure and the current collector respectively.
It should be understood that the term “intercalation” when used in relation to electroactive materials, particularly the silicon-containing materials, referred to herein includes a process where lithium is inserted into and disrupts the structure of the crystalline or amorphous silicon-containing material as well as a process in which lithium is dispersed between crystal planes defining the silicon-containing structure. The former process is more properly referred to as lithium insertion and is observed for materials comprising pure or substantially pure crystalline, amorphous and/or polycrystalline silicon. Some compounds or alloys of silicon will, however, also exhibit this form of behaviour. The dispersion of lithium between crystal planes within a crystalline or polycrystalline silicon-containing material is more often referred to as “intercalation” and is usually observed for materials comprising compounds or alloys of silicon.
In an attempt to overcome the stresses associated with bulk silicon anodes, anodes including silicon structures that are more easily able to accommodate the volume changes that occur on charging have been fabricated.
One of the earlier approaches employed anodes comprising pure silicon powder. Although it was expected that anodes fabricated from silicon powder would be better able to accommodate the volume expansion associated with lithium intercalation or insertion compared to bulk silicon electrodes, it was found that, in practice, these electrodes fared little better than bulk silicon electrodes and breakdown of the electronically conductive network due to the expansion of silicon powder particles was also observed.
In an attempt to improve the electronic contact between anode components during the charging and discharging of the cell, composite anodes comprising a mixture of powdered silicon and additional components such as a conductive material, a binder and optionally a further electroactive material were prepared. It was anticipated that these further components would be able to suppress and/or accommodate the large volume changes associated with the silicon species during the charging and discharging cycles of the cell. However, these electrodes were found to exhibit a reduced capacity compared with electrodes comprising silicon only and were unable to maintain this capacity over a required number of charging and discharging cycles.
In one prior art approach described by Ohara et al. (Journal of Power Sources 136 (2004) 303-306) which addresses the problems associated with the expansion and contraction of silicon during the charging and discharging cycles of the battery, silicon is evaporated onto a nickel foil current collector as a thin film and this structure is then used to form the anode of a lithium ion cell. However, although this approach gives good capacity retention, this is the case for only very thin films and thus the structures do not give usable amounts of capacity per unit area and increasing the film thickness to give usable amounts of capacity per unit area causes the good capacity retention to be eliminated due to mechanical breakdown as a result of the large volume expansion within the film.
Another approach used to address the problems associated with expansion of the silicon film is described in U.S. Pat. No. 6,887,511: Silicon is evaporated onto a roughened copper substrate to create medium thickness films of up to 10 μm. During the initial lithium ion insertion process the silicon film breaks up to form columns of silicon. These columns can then reversibly react with lithium ions and good capacity retention is achieved. However, the process does not function well with thicker films and the creation of the medium thickness film is an expensive process. Furthermore the columnar structure caused by the break-up of the film has no inherent porosity, which means that over time the pillars will, themselves, begin to crack and the electrode structure will likely not exhibit long term capacity retention.
In an attempt to overcome the problems associated with the bulk silicon, silicon powder and thin film silicon anodes described above, many workers have investigated alternative silicon and anode structures for the fabrication of anodes for lithium ion batteries. Examples of silicon structures investigated include arrays of silicon pillars formed on wafers and particles; silicon fibres, rods, tubes or wires; and complete porous particles comprising silicon. Anode structures having pores or channels formed therein have also been investigated.
U.S. Pat. No. 6,334,939 and U.S. Pat. No. 6,514,395 each disclose silicon based nano-structures for use as anode materials in lithium ion secondary batteries. Such nano-structures include cage-like spherical particles and rods or wires having diameters in the range 1 to 50 nm and lengths in range 500 nm to 10 μm. Similar nanostructures are disclosed in KR 1020027017125 and ZL 01814166.8. JP 04035760 discloses silicon based anode materials comprising carbon-coated silicon fibres having diameters in the range 10 nm to 50 μm for use in lithium ion secondary batteries. Batteries prepared using these nano-structures exhibited a total first cycle charging capacity of 1300 mAh/g and a reversible capacity of 800 mAh/g.
US 2007/0281216 discloses an anode active material for a lithium secondary battery comprising a mixture of silicon nano-particles, graphite, carbon black and a binder. The silicon nano-particles comprise either thread-like aggregates (a chain of connected spheroidal particles) having a primary particle size in the range 20 to 200 nm and a specific surface area of 11 m2/g or spherical particles having a primary particle size in the range 5 to 50 nm and a specific surface area of 170 m2/g. The silicon particles and threads are prepared using techniques such as chemical vapour deposition. Anodes exhibiting a capacity of up to 1000 mAh/g over 50 cycles are illustrated. The life of the battery is significantly increased if the battery is operated at a limited voltage level.
Polycrystalline silicon nano-wires and wires having cross-sectional diameters in the range 20 to 500 nm and aspect ratios of greater than 10, 50 or 100 and which have been prepared using epitaxial and non-epitaxial growth techniques are disclosed in U.S. Pat. No. 7,273,732.
Single crystalline silicon fibres, pillars or rods having diameters in the range 0.1 to 1 μm and lengths in the range 1 to 10 μm can also be prepared using lithographic and etching techniques as disclosed in U.S. Pat. No. 7,402,829. Alternative etching techniques such as those disclosed in WO 2007/083155, WO 2009/010758 and WO 2010/040985 can also be used.
The fibres, wires and rods described above are typically formed into a composite material containing, in addition to the silicon rods, wires and fibres, additional ingredients such as a binder, a conductive material and optionally a further electroactive material other than silicon. The composite material is also known as an anode mix and is typically used in the fabrication of anodes for lithium ion batteries. In accordance with the disclosure of the present inventors in WO 2009/010758 and WO 2009/010757 anode materials comprising silicon fibres or rods are preferably in the form of an entangled “felt” or “mat” in which silicon fibres are randomly connected with each other either directly or indirectly through the other components of the mix, and are also connected with the copper foil which acts as the current collector of the electrode.
By the term “felt or mat” it should be understood to mean a structure in which any one of the components of the structure is connected in a random or ordered manner with one or more other components of the structure so that there are multiple interconnections between the components. The mat may be provided in the form of a coating layer which is directly or indirectly applied, bonded or connected to a current collector or it may be in the form of a self-supporting structure, although this is less preferred. Preferably a felt or mat comprises one or more species of fibre as these help to strengthen the overall structure.
It has been observed by the present inventors that these felt structures produced using the silicon rod, wire and fibre products described above have an inherent porosity, (that is they contain voids or spaces between the fibres) as a result of the maximum attainable packing density of a random arrangement of fibres within a defined volume. These inherently porous electrodes were found to exhibit better capacity retention and cycling lifetimes compared to electrodes produced from bulk silicon, silicon powders and silicon films, for example. Without wishing to be constrained by theory, it is believed that the inherent porosity of these electrode structures provides at least some of the silicon components of the anode with space to expand into the voids or pores that are part of the electrode structure rather than push against each other during lithium intercalation or insertion (charging). The pores of the electrode are therefore able to accommodate the expansion of these silicon components during lithium intercalation or insertion within the volume initially occupied by the uncharged anode material, thereby reducing the volume increase within the electrode structure, the build up of stress and the application of pressure on the other cell components during the charging and discharging cycle As a result there will be less cracking of the silicon structures within the anode and a reduction in the extent of delamination of the electrode coating from the current collector, leading to better capacity retention and cycle-ability. The pores or voids also facilitate penetration of and therefore contact of the electrolyte with as much of the surface of the silicon material as possible during charging and discharging of the anode. This porosity is therefore believed to be important as it provides a path by which the lithium can be intercalated (or inserted) into the whole of the silicon material so that the lithiation of the silicon is as uniform as possible throughout the anode mass.
In addition to using silicon rods and fibres for the fabrication of porous electrode structures, it is also known to use silicon components which are themselves porous in the fabrication of porous electrodes or to form holes or channels into silicon based electrode structures having minimal porosity.
US 2009/0253033 discloses anode active materials having an inherent porosity for use in lithium ion secondary batteries. The anode material comprises silicon or silicon alloy particles with dimensions of between 500 nm and 20 μm and a binder or binder precursor. These particles are manufactured using techniques such as vapour deposition, liquid phase deposition or spraying techniques. During anode fabrication, the silicon/binder composite is heat treated to carbonise or partially carbonise the binder component thereby providing the anode with an inherent porosity. In a preferred embodiment the anodes of US 2009/0253033 include pores having dimensions in the range 30 nm to 5000 nm in order to accommodate the expansion of the silicon material during the charging and discharging phases of the battery. Anodes prepared using such silicon materials exhibit a capacity retention of from 70 to 89% and an expansion coefficient of 1 to 1.3.
Porous silicon anodes created by electrochemically etching channels into a silicon wafer have also been prepared. See, for example, H C Shin et al, J. Power Sources 139 (2005) 314-320. Electrolyte penetration was observed for channels having a pore diameter of 1 to 1.5 μm. It was observed that the peak current and the charge transferred during cyclic voltammetry increased with channel depth up to a limit. The amount of charge transferred for channels having an aspect ratio (channel depth to pore diameter) of the order of 1 was found to be only marginally less than those having an aspect ratio of 5. It was suggested that the channel walls were able to participate in the lithiation/dilithiation and that the presence of channels effectively increased the reactive area of the electrode. The porous structure remained essentially the same after a number of charge/discharge cycles despite the volume changes occurring as a result of the intercalation or insertion and release of lithium during these cycles. The channels created by electrochemical etching of a silicon wafer differ from the pores or voids created upon formation of a meshed electrode material using silicon fibres, wires and rods as described above in WO 2009/101758 and WO 2009/040985. The electrochemically etched electrode material is rigid and the entire volume of the electrode material will expand upon lithium intercalation or insertion. In contrast the voids within the meshed electrode material are able to contract and expand in response to the increase and decrease in the volume of the mesh comprising silicon components during lithium intercalation or insertion and release respectively. This means that silicon mesh type electrodes are more able to accommodate volume changes within the electrode structure upon lithium intercalation or insertion.
Rigid electrode structures such as those prepared by Shin et al tend to be associated with a build up of stress within the electrode structure on lithium intercalation or insertion as a result of the isotropic volume expansion of the entire electrode material. Providing the voids within the electrode structure are sufficiently open, the silicon mesh provides access for the electrolyte into the bulk of the electroactive anode. In contrast the more flexible meshed electrode structures including voids as described above are more able to accommodate expansion of the silicon material on lithium intercalation or insertion due the contraction and expansion of voids as described above. The overall expansion of a meshed electrode structure is therefore significantly less than that of the rigid channeled electrode structure described by Shin et al. This means that there will less build up of stress within meshed electrode structures compared to rigid electrode structures.
Porous silicon particles are also known and have been investigated for use in lithium ion batteries. The cost of manufacturing these particles is believed to be less than the cost of manufacturing alternative silicon structures such as silicon fibres, ribbons or pillared particles, for example. However, the life cycle performance of many of the composite electrodes prepared to date, which comprise porous silicon particles needs to be significantly improved before such electrodes could be considered to be commercially viable.
Porous silicon particles having dimensions in the range 4 to 11 μm, an average pore sizes of 6 to 8 Å and a BET surface area of from 41 to 143 m2/g have been prepared for use in fields such as drug delivery and explosive design (Subramanian et al, Nanoporous Silicon Based Energetic Materials, Vesta Sciences NJ 08852 Kapoor and Redner, US Army RDE-COM-ARDEC Picatinny Arsenal NJ 07806, Proceedings of the Army Science Conference (26th) Orlando, Fla., 1-4 Dec. 2008). There is no indication in Subramanian et al that their silicon containing porous particles would be suitable for use in the fabrication of lithium ion batteries.
Silicon nanosponge particles having a network of pores extending through the particle structure have also been prepared, U.S. Pat. No. 7,569,202. Nanosponge particles having a diameter of 1 to 4 μm and pore diameters of 2 to 8 nm are prepared by stain etching metallurgical grade silicon powders to remove both silicon material and impurities. It is believed that the impurities in the metallurgical grade silicon are preferentially etched away to give particles having a network of pores distributed throughout. The nanosponge particles can be surface treated to introduce functional groups onto the silicon surface. U.S. Pat. No. 7,569,202 teaches that the surface functional groups enable the nanosponge particles to be used for a broad range of applications from drug delivery to explosives. U.S. Pat. No. 7,569,202 does not teach the application of nanosponge particles in lithium ion batteries.
U.S. Pat. No. 7,244,513 discloses a partially porous silicon powder comprising silicon particles having a solid silicon core and an outermost layer of porous silicon. These partially porous silicon particles are prepared by stain etching particles having a dimension in the range 1 μm to 1 mm to give partially porous particles having a porous outer shell in which the pore dimensions in the range 1 nm to 100 nm. The partially porous particles are then subjected to ultrasonic agitation to give silicon nanoparticles having a dimension in the range 10 nm to 50 nm. U.S. Pat. No. 7,244,513 teaches that the nanoparticles could be used in applications such as sensors, floating gate memory devices, display devices and biophysics. There is no suggestion that these nanoparticles could be used in the fabrication of a lithium ion battery.
US 2004/0214085 discloses an anode material comprising an aggregate of porous particles that is capable of withstanding pulverization during the charging and discharging cycles of the battery. According to US 2004/0214085, the reason why the particles are able to withstand pulverisation is because the external volume of the porous particle is maintained during the charging and discharging cycle of the battery due to compression of the particle voids when the particle expands during the process of intercalating lithium ions into silicon. The porous particles in the aggregate have an average particle size in the range 1 μm to 100 μm and pore sizes in the range 1 nm to 10 μm. For particles having diameters of less than 1 μm the relative volume of the pores within the particle is excessive and the hardness of the particle is compromised. Particles having a diameter of more than 100 μm are unable to accommodate the volume changes associated with the intercalation or insertion and deintercalation or release of lithium and cannot prevent pulverisation of the particle. The particles are prepared by quenching an alloy of silicon with another element, M to form a quenched alloy particle comprising an amorphous silicon phase and an element, M, which can be eluted from the particle to provide a porous particle. 50:50 and 80:20 silicon-nickel alloys and 70:30 Al:Si alloys were used to prepare alloy containing particles using a gas atomisation technique in which a helium gas pressure of 80 kg/cm2 and a quenching rate of 1×105 K/s was used. The quenched particles were washed in acid (H2SO4 or HCl) to remove either the Ni or the Al to give porous particles, which contained a mixture of both amorphous and crystalline silicon. Batteries prepared using the Si porous materials of US 2004/0214085 have a capacity retention of between 83 and 95% over 30 cycles.
The porous particle of US 2004/0214085 is characterised by the ratio of the pore diameter, n, to the particle diameter, N and the volume ratio of the voids to the porous particle. n/N is preferably in the range 0.001 to 0.2 so that the diameter of the pores within the particles is very small in order that the hardness of the particle can be maintained. The volume ratio of the voids to the porous particle is preferably in the ratio 0.1% to 80% so that the expansion and contraction of the silicon volume during intercalation or insertion and deintercalation or release of lithium is fully compensated by the voids, the entire volume of the porous particle is maintained and the particles are not degenerated.
U.S. Pat. No. 7,581,086 discloses an electrode material comprising porous silicon particles, which particles are prepared by quenching a eutectic alloy of silicon and another metal (typically aluminium) using a roll solidification method at a cooling rate of greater than 100K/s to give a thin film alloy sheet. The thin film is pulverised to give alloy particles having a typical diameter of 15 μm, which are typically etched in HCl to give porous Si particles. Electrode materials prepared from these powder particles exhibited a capacity retention of approximately 68% at 10 cycles.
US 2009/0186267 discloses an anode material for a lithium ion battery, the anode material comprising porous silicon particles dispersed in a conductive matrix. The porous silicon particles have a diameter in the range 1 to 10 μm, pore diameters in the range 1 to 100 nm (preferably 5 nm), a BET surface area value in the range 140 to 250 m2/g and crystallite sizes in the range 1 to 20 nm. The porous silicon particles are mixed with a conductive material such as carbon black and a binder such as PVDF to form an electrode material, which can be applied to a current collector (such as a copper foil) to give an electrode. Although US 2009/0186267 suggests that these materials could be used for the manufacture of a battery, there is no data in this document to suggest that a battery has actually been manufactured.
Kim et al teaches the preparation of three-dimensional porous silicon particles for use in high performance lithium secondary batteries in Angewandte Chemie Int. Ed. 2008, 47, 10151-10154. Porous silicon particles are prepared by thermally annealing composites of butyl capped silicon gels and silica (SiO2) nano-particles at 900° C. under an argon atmosphere and etching the silica particles out of the annealed product to give a carbon coated porous amorphous silicon particle having a pore wall thickness of 40 nm, pore diameters of the order of 200 nm and an overall particle size of greater than 20 μm. Silicon crystallites having a diameter of less than 5 nm were observed within the structure. Half cells prepared using these amorphous porous particles exhibited improved first cycling efficiency, which was thought to be due to the carbon coating. It was also suggested that an amorphous silicon structure could act as a buffer against the expansion of crystalline silicon upon intercalation or insertion.
Although anode structures comprising silicon fibres, rods and wires have been found to exhibit both a better capacity retention and an improved cycle life compared to bulk silicon and silicon powder anodes, an improvement in their absolute capacity and cycle life is still desired. Depending on the shape and dimension of the silicon elements, there can be a limit to the achievable packing density in the composite mix which can restrict the maximum achievable electrode capacity. Furthermore, the methods and costs associated with the manufacture of these silicon structures needs to be further refined and reduced respectively. Even with inherent porosity, electrode structures comprising silicon fibres, rods and wires have been observed to exhibit an effect known as “heave” in which the bulk of the silicon electrode material expands away from the surface current collector during intercalation, which may result in delamination. This bulk does appear to survive the heave process and is able to substantially resume its original configuration on release of the lithium from the silicon fibres, but it exerts pressure on other cell components during cycling.
Further it has been found to be difficult to prepare anode structures comprising porous silicon particles that are able to provide adequate performance in terms of absolute capacity, capacity retention and cycle-ability. Anode structures comprising porous particles of diameter less than 500 nm, for example, do not exhibit good capacity characteristics because the particle pores are generally too small to facilitate electrolyte penetration and efficient intercalation or insertion and release of lithium into the silicon structure. Further, the small particles tend to agglomerate within the electrode structure, which leads to delamination over a number of charging and discharging cycles. In addition because the pore wall thickness (average thickness of material separating any one void or pore within a particle structure from its adjacent pore or void) of these particles tends to be very low (less than 50 nm), their associated surface area tends to be high. A high surface area is associated with significant first cycle losses of lithium in the electrode structure due to the formation of an excessive Solid Electrolyte Interphase layer (SEI) as a result of the consumption of lithium in the formation of these layers. Particles containing pores of a sufficiently large size to accommodate electrolyte penetration and which have thicker pore walls of 0.1 to 2 μm tend themselves to have diameters that are too large to be successfully accommodated into an electrode structure having an overall uniform thickness of around 50 μm.
The fibres or wires used in the formation of silicon mesh electrode structures are also believed to have a high surface area to volume ratio. These mesh like electrode structures are also believed to be associated with high first cycle losses for the reasons given above.
It will be appreciated from the foregoing that the majority of approaches used to date for creating porous particles result in the production of approximately spheroidal-shaped particles with relatively smooth curved surfaces. Such shapes are not ideal for creating networks of electronically connected particles in an electrode. This is because the surface area of contact between one spheroidal particle and another or between one spheroidal particle and a conductive additive particle is small; this means that the electronic conductivity throughout the connected mass of active particles is relatively low, reducing performance.
Many of the electrodes produced using the electro-active silicon materials discussed herein above are not able to exhibit the characteristics of uniform thickness, homogeneity and porosity. Such electrodes do not comprise a strongly connected network of active particles that are able to accommodate the expansion and contraction of the silicon material into its own volume without cracking or de-lamination during the charging cycles of the battery.
There is a need, therefore, for an electroactive material and an electrode structure that addresses the problems associated with the silicon based electrodes outlined above.